Computer Architecture: Branch Prediction and Hazards
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Questions and Answers

What is a necessary condition for two instructions to be paired together in the decode stage?

  • They must lack dependencies. (correct)
  • They must both contain displacements.
  • They must both be floating point instructions.
  • They must be dependent on each other.

What happens if two paired instructions are executing concurrently and one stalls due to hazard control?

  • Both instructions will stall. (correct)
  • The other instruction will continue executing.
  • Only the stalled instruction will halt execution.
  • The processor will discard both instructions and begin again.

What is the primary function of the branch target buffer?

  • To store immediate values for instructions.
  • To hold information about all branches. (correct)
  • To execute the branch instructions.
  • To manage data dependencies.

In the context of branch prediction, what does 'State 0' indicate?

<p>A branch is very unlikely to jump. (B)</p> Signup and view all the answers

Under what condition will a branch move from State 0 to State 2 in the branch prediction mechanism?

<p>If the branch jumps two times in a row. (D)</p> Signup and view all the answers

What would happen to the pipeline if there is a wrong branch prediction?

<p>It would flush the pipes and wastes cycles. (D)</p> Signup and view all the answers

What is notably incorrect about the Pentium's branch prediction algorithm as described in the text?

<p>State 0 will jump directly to State 3. (C)</p> Signup and view all the answers

What is the initial prediction for a branch that is not found in the branch target buffer?

<p>It will not jump (A)</p> Signup and view all the answers

When does a branch receive its first entry in the branch target buffer?

<p>The first time the branch jumps (C)</p> Signup and view all the answers

What is the primary reason later Pentium models changed their branch prediction mechanism?

<p>To increase the accuracy of branch predictions (A)</p> Signup and view all the answers

Why is a simple two-bit counter insufficient for recognizing some loop patterns?

<p>It cannot store enough previous branch states (B)</p> Signup and view all the answers

What is the role of the 4-bit register in later Pentium branch prediction?

<p>To record the previous behavior of the branch (B)</p> Signup and view all the answers

If the 4-bit register's value is 0001, what does this indicate?

<p>The branch jumped only the last time in 4 attempts (B)</p> Signup and view all the answers

What is the purpose of the 16, 2-bit counters in the improved prediction mechanism?

<p>To predict the next value for specific branch trends (B)</p> Signup and view all the answers

How is the appropriate 2-bit counter selected for a given branch prediction?

<p>Based on the current value of the 4 bit register (D)</p> Signup and view all the answers

What is the main benefit of combining a 4-bit register with 16 updated 2-bit counters?

<p>More accurate branch prediction (B)</p> Signup and view all the answers

What is the primary function of the Next IP unit within the Instruction Fetch Unit (IFU)?

<p>To provide the index for the I-cache. (B)</p> Signup and view all the answers

Which of the following best describes the branch prediction scheme employed in the Pentium II/III microarchitecture?

<p>Two-level adaptive scheme. (D)</p> Signup and view all the answers

What is the primary role of the Branch Target Buffer (BTB) in the Pentium II/III processor?

<p>To maintain branch history and predicted target addresses. (B)</p> Signup and view all the answers

Which unit is responsible for translating complex instructions into simpler micro-operations?

<p>The Microcode Instruction Sequencer. (B)</p> Signup and view all the answers

Where does the instruction fetch unit (IFU) access its instruction data?

<p>The non-blocking I-cache. (D)</p> Signup and view all the answers

What is the primary function of the Return Address Stack?

<p>To predict return addresses for procedure calls. (D)</p> Signup and view all the answers

What is the purpose of P4 branch hints?

<p>To provide information to software to improve trace formation. (B)</p> Signup and view all the answers

How does Out-of-Order Execution enhance performance?

<p>By prioritizing the most common operations in context. (A)</p> Signup and view all the answers

What is the role of the trace cache?

<p>To store micro-ops generated from instructions. (B)</p> Signup and view all the answers

What does 'double-pumped ALU' refer to?

<p>An ALU that executes an operation on each edge of a clock cycle. (C)</p> Signup and view all the answers

What is the function of the retirement stage in the execution pipeline?

<p>To manage and organize completed micro-ops while ensuring the correct order. (A)</p> Signup and view all the answers

What are RAT and ROB used for within register renaming?

<p>To map architectural and physical registers, and to organize completed micro-ops. (B)</p> Signup and view all the answers

How does the L1 Instruction Cache reduce decoder latency?

<p>By storing traces of executed code, directly fetching decodes instructions. (A)</p> Signup and view all the answers

What does it mean for the L1 Data Cache to be non-blocking?

<p>It can handle multiple outstanding load misses, as many as 4. (A)</p> Signup and view all the answers

What kind of replacement algorithm is used for the on-chip caches?

<p>A pseudo-LRU (Least Recently Used) algorithm. (C)</p> Signup and view all the answers

What does a confidence estimator attempt to assess in branch prediction?

<p>The prediction made by the branch predictor. (D)</p> Signup and view all the answers

What characterizes a branch with low confidence?

<p>It changes its direction frequently and irregularly. (B)</p> Signup and view all the answers

Which of the following is NOT a classification category for branch outcomes based on prediction and confidence?

<p>Unpredicted with high confidence U(HC). (C)</p> Signup and view all the answers

How can saturation counter information be used in constructing a confidence estimator?

<p>To speculate more aggressively when the confidence level is higher. (D)</p> Signup and view all the answers

What is a miss distance counter (MDC) table used for in branch prediction?

<p>To determine if a branch has high or low confidence. (A)</p> Signup and view all the answers

In a PAs predictor scheme, when do small numbers of branch history patterns typically lead to correct predictions?

<p>When the patterns match a previously seen pattern. (A)</p> Signup and view all the answers

What is one way that confidence estimation can be used beyond branch prediction?

<p>Thread switching in multithreaded processors. (D)</p> Signup and view all the answers

What is the function of predicate registers in predicated instructions?

<p>To store the conditional execution result. (A)</p> Signup and view all the answers

When does a predicated instruction execute if its predicate is resolved to true?

<p>Only if its predicate is true will execution happen. (D)</p> Signup and view all the answers

What happens to a predicated instruction if its predicate is false?

<p>It is discarded and does not execute. (C)</p> Signup and view all the answers

Flashcards

Instruction Pairing Restrictions

Two instructions cannot be paired in the Decode stage if they have dependencies or involve complex addressing modes like displacements or immediate addressing.

Branch Prediction

A technique used by the Pentium processor to speed up execution by predicting the outcome of branch instructions. This prediction is based on the branch's past behavior.

Branch Target Buffer

The Pentium processor utilizes a branch target buffer that stores information about all branches. This buffer helps in predicting branch behavior based on past execution patterns.

Branch Prediction States

The branch prediction mechanism uses four states to track a branch's tendency to jump or not jump. These states represent the likelihood of a branch jumping.

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Pentium Branch Prediction Algorithm (Incorrect)

The branch prediction algorithm in Pentium, where State 0 transitions directly to State 3, is believed to be incorrect. This transition bypasses the usual progression through States 1 and 2.

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Branch Prediction State 0

If a branch does not jump for two consecutive times it will go down to State 0.

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Branch Prediction State 2

If a branch jumps for two consecutive times it will go from State 0 to State 2.

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State 0

The state where a branch has never been executed before. The branch prediction mechanism assumes it won't jump.

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State 1

The state where a branch jumped at least once but not consecutively. The prediction algorithm assumes the branch won't jump.

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State 2

The state where a branch jumped at least twice consecutively. The prediction algorithm assumes the branch will jump.

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State 3

The state where a branch jumped at least three times consecutively. The prediction algorithm assumes the branch will jump.

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Two-Bit Branch Prediction

A technique used to improve branch prediction accuracy, where a two-bit counter is used to record the recent execution history of a branch.

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Four-Bit Branch Prediction

An improvement upon two-bit branch prediction, where a 4-bit register is used to track the previous state of the branch, combined with 16 two-bit counters that store historical data.

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Loop Prediction

A type of branch prediction that helps recognize and predict common loop patterns in code.

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Pentium Branch Prediction

A branch prediction scheme implemented in later Pentium processors (MMX, Pentium Pro, Pentium II) that combines a history register with a set of counters for improved prediction accuracy.

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Branch Prediction in Pentium II/III

The Pentium II/III processor uses a two-level adaptive scheme for branch prediction, known as the Yeh and Patt algorithm. This scheme involves a Branch Target Buffer (BTB) that stores branch history information and predicted target addresses.

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Branch Target Buffer (BTB)

The Branch Target Buffer (BTB) is a special memory within the Pentium II/III processor that stores information about recently encountered branches, including their target addresses and past behavior.

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Next IP Unit (Pentium II/III)

The Next Instruction Pointer (Next IP) unit in the Pentium II/III processor provides the I-cache index, trap/interrupt status, and branch misprediction indications. It's essentially a guide for fetching the next instruction.

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Instruction Fetch Unit (IFU)

The Instruction Fetch Unit (IFU) in the Pentium II/III processor accesses a non-blocking instruction cache (I-cache). This allows the processor to fetch instructions efficiently without waiting for the previous ones to be completed.

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Non-Blocking I-cache in Pentium II/III

The Pentium II/III processor utilizes a non-blocking I-cache within its Instruction Fetch Unit (IFU). This allows the processor to continue fetching instructions without waiting for the previous ones to be completely processed, improving efficiency.

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Confidence Estimator

A branch prediction technique that assesses the accuracy of branch predictions, categorizing them as high or low confidence.

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Low Confidence Branch

A branch whose direction changes frequently and unpredictably, making its outcome difficult to predict.

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Correctly Predicted with High Confidence (C(HC))

A branch that is predicted correctly and with a high level of certainty.

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Correctly Predicted with Low Confidence (C(LC))

A branch that is predicted correctly, but with a low level of certainty.

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Incorrectly Predicted with High Confidence (I(HC))

A branch that is predicted incorrectly, but with a high level of certainty.

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Incorrectly Predicted with Low Confidence (I(LC))

A branch that is predicted incorrectly and with a low level of certainty.

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Confidence Estimation

A method for determining the confidence level of a branch prediction based on the history of the branch.

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Predicated Instructions

Instructions that are executed only if a specific condition is met. They use predicate registers to store the Boolean result of a condition check.

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Predicate Registers

Registers that store the Boolean result of a condition check. They determine whether a predicated instruction is executed or discarded.

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Predicated Instruction Execution

The process by which predicated instructions are processed in the instruction pipeline. The pipeline can speculatively execute instructions before their conditions are resolved.

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L1 Instruction Cache (Trace Cache)

A special memory that stores decoded instructions, reducing the time it takes to execute loops and improve performance.

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Return Address Stack

A memory component storing return addresses used to track procedure calls. It guarantees that the processor can quickly return to the correct location after a function call.

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Register Renaming

A mechanism that renames registers, allowing for multiple instructions to access the same register without conflict. It optimizes execution by reducing bottlenecks.

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Out-of-Order Execution

A technique used to optimize performance by executing instructions out of order, if dependent instructions are available. This allows the processor to make better use of its resources and execute programs faster.

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Execution Unit

A unit that handles the execution of instructions. There are different execution units responsible for specific operations, such as floating-point calculations, memory access, and logical operations.

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L1 Data Cache

A memory component that stores data frequently used by the processor. It allows for faster access to data compared to main memory, improving overall performance.

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Issue

Instructions fetched from memory and decoded by a translation engine, ready for execution.

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Micro-operations (µops)

Instructions are broken down into smaller units called micro-operations (µops). This allows for more efficient execution by dividing complex operations into simpler steps.

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Study Notes

Intel Pentium Processors

  • The Intel Pentium processors are a family of microprocessors that have evolved through several generations.
  • Different generations used varying microarchitectures like P6 and NetBurst.
  • These processors have advanced functionalities and technologies like MMX, SSE, and Hyper-Threading Technology.
  • The processors have varying instruction sets and are designed for different applications like general purpose computing and multimedia.

Outline of Topics

  • Introduction to Intel Architecture
  • Willamette Processor (11/2000)
  • Instruction Set Architecture (IA-32)
  • Instruction Stream
  • Data Stream
  • Issues/Problems in Early Pentium Processors
  • Pentium 4 Revisions: Northwood, Xeon (Prestonia), Prescott
  • Dual Core Processors and Smithfield
  • IA-32 (cont'd), Addressing modes and SIMD instruction sets (MMX, SSE)
  • Pentium Pipelined Integer Unit stages (Pre-fetch, Decode, Address generate, Execution, Writeback)
  • Superscalar architecture of Pentium Processor
  • Branch Prediction: the concept and implementation in Pentium Processors
  • Branch Target Buffer (BTB) and its function
  • Branch prediction algorithms in later Pentium models
  • Data types for MMX technology
  • SIMD execution model
  • P6 family processors and time frame (1995-1999)
  • Pentium Pro processor
  • Dynamic execution and micro-data flow analysis
  • Cache enhancements
  • New Generation Chips (Pentium MMX)
  • Superpipelining and Superscalar
  • MMX (Multimedia Extensions) features
  • Pentium III processor and SSE (Streaming SIMD Extensions)
  • Pentium 4 processor and NetBurst micro-architecture.
  • Pentium 4 Processor Family (2000-2005)
  • NetBurst Micro-Architecture, design and components
  • Streaming SIMD Extensions 2 (SSE2)
  • Streaming SIMD Extensions 3 (SSE3) and different formats of data types
  • Hardware support for SSE2
  • SSE2 instructions (part 1&2)
  • Pentium 4 instruction stream
  • Fetching and Decoding Instructions in instruction stream: use of Trace cache
  • Methods of prefetching
  • Tracing Cache Function
  • Branch Prediction in P4 Architecture
  • Return Address Stack
  • Out-of-Order Execution
  • Execution Units (ALU, Load, Store)
  • In-Order Retirement Section
  • Pentium II/III Pipeline components, diagrams and details
  • Pentium II/III summary and offsprings
  • First-level caches & functionality
  • Second-level caches and features
  • 400 MHz Intel NetBurst micro-architecture System Bus
  • Intel Celeron Processor family features, evolution
  • Overclocking Celeron Processors
  • Hybrid Branch Predictors
  • Branch prediction algorithms simulations by Grunwald.
  • Predicates and Multipath Execution (Confidence Estimation), implementation details
  • Eager (Multipath) Execution, implementation details
  • Prediction of Indirect Branches
  • Branch handling techniques and examples
  • High-bandwidth branch prediction
  • Different Versions of Intel Xeon Processors: core features and advancements as servers for high performance computing
  • Pentium M Processor (2003-2005) and it's purpose for low power computing for mobile devices
  • Pentium Processor Extreme Edition (2005) and its dual-core technology advances including hyper-threading technology and new advanced features
  • Pentium III vs. Pentium 4 Pipeline
  • Comparison between Pentium 3 and Pentium 4 processors (Table format including relevant data points)
  • Execution benchmarks (using MPEG4, Quake III Arena... on Pentium and related results)
  • Comparison between Celeron and Duron Processors and their respective results from benchmarks

Key Facts and Entities

  • IA-32 architecture
  • MMX (Multimedia Extensions)
  • SSE (Streaming SIMD Extensions), SSE2, SSE3
  • Hyper-threading technology
  • NetBurst micro-architecture
  • Trace cache
  • Dynamic execution
  • Various cache levels
  • Pentium MMX, Pentium II, Pentium III, Pentium 4, and Intel Xeon processors.
  • Pentium Pro, Pentium II, Pentium III
  • Different components of the processors like Execution units, Memory blocks, Caches, pipelines and their functionalities
  • Different benchmarking techniques

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Description

This quiz delves into the essential concepts of branch prediction and hazard control in computer architecture. It covers various scenarios involving instruction pairing, stalling instructions, and the functionality of the branch target buffer. Test your understanding of how these elements affect pipeline efficiency and performance.

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